Possibility of LDL--collagen complex formation was investigated in vitro by biochemical assay and electron microscopy. Types I and III collagen isolated from bovine thoracic aorta were incubated with human low density lipoproteins (LDL) at physiological ionic strength, pH and temperature. Biochemical quantification showed that 10-20 micrograms LDL (cholesterol) were bound per 100 micrograms collagen, binding of type III being slightly more pronounced (17%) than that of type I (11%). Binding was in inversely proportional to the extent of fibrillation. The increase of ionic strength and pH reduced the binding, indicating the electrostatic nature of the interaction. These observations suggest a possible trapping mechanism of LDL in the extracellular matrix by means of collagen, which may be relevant for the development of the atherosclerotic lesions.